Classically, thermal spraying has used high temperature flames or plasmas to melt and project metal and other particles against a surface to be coated. The molten particles flatten upon impact against the workpiece, adhere to the workpiece, and then contract due to cooling. The final cooled coating is characterized by tensile stresses as the individual "splats" are restrained by the workpiece and adjacent adhering particles from reaching a state of zero stress. Such coatings cannot be built-up to appreciable thicknesses -with most coatings limited to under 1/32-inch.
More recently, a new thermal spraying technique has been introduced. It speeds heated solid particles to such great speed that upon impact they fuse to form a dense coating. The applicant has found that such coatings are under compressions--just the opposite of coatings produced by impact of molten particles. This impact-fusion method allows much thicker coatings to be produced. Yet, a point is reached where the overall compressive faces produced in the coating can lead to coating cracking or separation from the workpiece surface.
Although compressive stressing of thermal spray coatings is to be preferred to tensile stressing, the optimum coating (for most cases) would be zero stress levels.